1. Field of the Disclosure
Embodiments disclosed herein relate generally to threaded connections. More particularly, embodiments disclosed herein relate to two-step wedge thread connections and related methods of makeup.
2. Background Art
Casing joints, liners, drill pipe, and drill collars (collectively referred to as “tubulars”) are often used in drilling, completing, and producing a well. Casing joints, for example, may be emplaced in a wellbore to stabilize a formation, to protect a formation against elevated wellbore pressures (e.g., wellbore pressures that exceed a formation pressure), and the like. Casing joints may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections may be designed so as to form a seal between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomeric seal (e.g., an o-ring seal), a metal-to-metal seal formed proximate the connection, or similar seals known in the art. In some connections, seals are formed between the internal and external threads. Connections with this characteristic are said to have a “thread seal.” As used herein, a “thread seal” means that a seal is formed between at least a portion of the internal thread on the box member and the external thread on the pin member.
It will be understood that certain terms are used herein as they would be conventionally understood where tubular joints are being connected in a vertical position along a central axis of the tubular members such as when making up a pipe string for lowering into a well bore. Thus, the term “load flank” designates the side wall surface of a thread that faces away from the outer end of the respective pin or box member on which the thread is formed and supports the weight (i.e., tensile load) of the lower tubular member hanging in the well bore. The term “stab flank” designates the side wall surface of the thread that faces toward the outer end of the respective pin or box member and supports forces compressing the joints toward each other such as the weight of the upper tubular member during the initial makeup of the joint or such as a force applied to push a lower tubular member against the bottom of a bore hole (i.e., compressive force). The term “face” of the box is the end of the box member facing outward from the box threads and the term “nose” of the pin is the end of the pin member facing outward from the threads of the connection. Upon makeup of a connection the nose of the pin is stabbed into and past the face of the box.
One type of thread commonly used to form a thread seal is a wedge thread. In FIGS. 1A and 1B, a connection 100 having a wedge thread is shown. “Wedge threads” are characterized by threads that increase in width (i.e., axial distance between load flanks 125 and 126 and stab flanks 132 and 131) in opposite directions on the pin member 101 and box member 102. Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference.
On the pin member 101, the pin thread crest 122 is narrow towards the distal end of the pin member 101 while the box thread crest 191 is wide. Moving along the axis 105 (from right to left), the pin thread crest 122 widens while the box thread crest 191 narrows. The rate at which the threads change in width along the connection is defined by a variable known as the “wedge ratio.” As used herein, “wedge ratio,” although technically not a ratio, refers to the difference between the stab flank lead and the load flank lead, which causes the width of the threads to vary along the connection. Furthermore, as used herein, a thread “lead” refers to the differential distance between a component of a thread on consecutive threads. As such, the “stab lead” is the distance between stab flanks of consecutive thread pitches along the axial length of the connection. In FIGS. 1A and 1B, the thread surfaces are tapered, meaning that the pin thread 106 increases in diameter from beginning to end while the box thread 107 decreases in diameter in a complimentary manner. Having a thread taper improves the ability to stab the pin member 101 into the box member 102 and distributes stress in the connection.
For wedge threads, a thread seal is accomplished by the contact pressure caused by interference over at least a portion of the connection between the pin load flank 126 and the box load flank 125 and between the pin stab flank 132 and the box stab flank 131, which occurs when the connection is made-up. Close proximity or interference between the roots 192 and 121 and crests 122 and 191 completes the thread seal when it occurs over at least a portion of where the flank interference occurs. Higher pressure may be contained with increased interference between the roots and crests (“root/crest interference”) on the pin member 101 and the box member 102 and by increasing flank interference. This particular connection also includes a metal-to-metal seal that is accomplished by contact between corresponding sealing surfaces 103 and 104 located on the pin member 101 and box member 102, respectively.
A property of wedge threads, which typically do not have a positive stop torque shoulder on the connection, is that the make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member varies more for a given torque range to be applied than connections having a positive stop torque shoulder. As used herein, “make-up” refers to threading a pin member and a box member together. A final make-up refers to threading the pin member and the box member together up to a desired amount of torque, or based on a relative position (axial or circumferential) of the pin member with the box member.
For wedge threads that are designed to have both flank interference and root/crest interference at a selected make-up, both the flank interference and root/crest interference increase as the connection is made-up (i.e. increase in torque increases flank interference and root/crest interference). For wedge threads that are designed to have root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks and corresponding roots and crests come closer to each other (i.e. clearance decreases or interference decreases) during make-up.
Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the torque on the connection. Thus, a wedge thread may be able to thread seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the torque on the connection; however, this also increases stress on the connection during make-up, which could lead to failure during use.
Prior to make-up a flowing joint compound commonly referred to as “pipe dope” is typically applied to surfaces of a threaded connection to improve the thread seals and provide lubrication during make-up of the connection. For example, the base (e.g., a grease) of the pipe dope may assist a wedge-threaded connection in achieving a thread seal between load and stab flanks thereof, e.g., as disclosed in U.S. Pat. No. RE 34,467 issued to Reeves. Further, pipe dope may contain metallic particle additives, such as copper to protect the threads of the pin and box members from friction galling during make-up and break-out.
When a wedge thread connection is made-up, excess pipe dope may become trapped (rather than being squeezed out) between engaging pin and box threads, which may either cause false elevated torque readings (leading to insufficient make-up or “stand-off”) or, in certain circumstances, damage the connection. Pipe stand-off due to inadequate evacuation of the pipe dope is detrimental to the structural integrity of wedge thread connections. As the pressure build-up may bleed off during use, the connection is at risk of accidentally backing off during use. Therefore, stand-off in wedge thread connections is of particular concern as it may lead to loss of seal integrity or even mechanical separation of the two connected members.
FIG. 2 shows a prior art two-step connection 150. The threads that form the connection are separated on two different “steps,” a large step indicated by the bracket 31 and a small step indicated by the bracket 32. The portion between the large step 31 and the small step 32 is commonly referred to as a mid-step 901. In some connections, the mid-step 901 may be used as a metal-to-metal seal. The pin thread crest 222 on the small step 32 of the pin member 101, at its full design height, does not interfere with the box thread crest 221 on the large step 31 of the box member 102 when the pin member 101 is stabbed into the box member 102. The diameter of the small step 32 of the pin member 101 is smaller than the smallest crest-to-crest thread diameter on the large step 31 of the box member 102. The pin thread 106 on the small step 32 can be stabbed past the box thread 107 on the large step 31. The threads on both the small step 32 and the large step 31, which have substantially the same nominal lead, engage with each revolution to make-up the connection. Thus, the number of revolutions during which the threads slide or rub against each other is reduced for the same number of engaged threads. A two-step connection allows for each of the steps to have threads with different characteristics as long there is little or no variance in the nominal lead of the threads on the steps.
A two-step wedge thread connection is disclosed in U.S. Pat. No. 6,206,436 issued to Mallis and assigned to the assignee of the present invention. That patent is incorporated herein by reference. Mallis discloses a two-step wedge thread connection having different wedge ratios, one of which is considered to be an “aggressive” wedge ratio and the other a “conservative” wedge ratio. “Aggressive” refers to the larger wedge ratio, and “conservative” refers to the smaller wedge ratio. Everything else the same, the greater the wedge ratio, the more determinate the make-up. Too large of a wedge ratio may have an inadequate wedging effect, which can allow the connection to back-off during use. Smaller wedge ratios are better able to resist backing-off of the connection. Too small of a wedge ratio may have such an indeterminate make-up that galling may occur over the lengthened make-up distance. Mallis discloses that one of the steps can have a wedge ratio that is optimized for a more determinate make-up (aggressive), while the other step can have a wedge ratio that is optimized for preventing back-off of the connection (conservative).
FIGS. 3A and 3B show cross-section views of a conventional two-step wedge thread connection 200 prior to a final makeup. The connection 200 includes a pin member 201 having pin wedge threads 106 thereon and a box member 202 having corresponding box wedge threads 107 thereon. Further, the connection 200 has first step 31 and second step 32, with a mid-step region 901 located therebetween. As shown, an axial separation of the two wedge thread steps 31, 32 of the pin member 201, indicated by distance ‘B,’ is substantially equal to an axial separation of the two wedge thread steps 31, 32 of the box member 202, indicated by distance ‘A.’ Thus, the pin wedge threads 106 on the first step 31 of the pin member 201 may be characterized as “in-phase” with the box wedge threads 107 on the first step 31 of the box member 202. Likewise, the pin wedge threads 106 on the second step 32 of the pin member 201 may be characterized as “in-phase” with the box wedge threads 107 on the second step 32 of the box member 202.
The corresponding pin and box threads on the two steps 31, 32 are in-phase such that during makeup, gaps 137 between approaching load flanks 125, 126 are equal to gaps 138 between approaching stab flanks 131, 132 on the first step 31. Similarly, gaps 137 between approaching load flanks 127, 128 are equal to gaps 138 between approaching stab flanks 133, 134 on the second step 32. Thus, corresponding load flanks 125, 126 and stab flanks 131, 132 on first step and corresponding load flanks 127, 128 and stab flanks 133, 134 on second step 32 will contact at substantially the same time (i.e., at final makeup). FIGS. 3C and 3D illustrate the conventional two-step wedge thread connection 200 at a final makeup. Because the corresponding load flanks 125, 126 and stab flanks 131, 132 on first step 31, and corresponding load flanks 127, 128 and stab flanks 133, 134 contact at substantially the same time, the interference generated between the surfaces is substantially equal.
A threaded connection having improved make-up and break-out torque characteristics would be appreciated by those skilled in the art.